To date, copolyester compositions comprising terephthalic acid or an ester thereof or mixtures thereof, 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD) residues and 1,4-cyclohexanedimethanol (CHDM) residues have been focused primarily for use in the injection molding and extrusion of amorphous articles even in relatively thick parts due to slow crystallization rates. These slow crystallization rates are achieved by modifying the polyester (PCT), which is based on terephthalic acid (TPA) or ester thereof such as dimethyl terephthalate (DMT), or mixture thereof, and CHDM (70/30 trans/cis) with TMCD at levels greater than 20 mole percent of the diol fraction.
On the other hand, unmodified PCT is known to crystallize extremely fast making it extremely difficult to mold or extrude even thin parts without thermal crystallization. Thermal crystallization of PCT and other polyesters typically leads to opacity in parts.
In order to slow down the crystallization rate of PCT, additional dicarboxylic acids or glycols can be used to modify PCT in order to slow down the crystallization rate. In particular, ethylene glycol or isophthalic acid-modified PCTs are known in the art and are commercially available. Polyethylene terephthalate (PET) or slightly modified PET's have been proven useful in clear semi-crystalline articles such as soft drink bottles, oriented films, and oriented fibers. These articles from PET are prepared by processes that take advantage of PET's ability to strain induce crystallize above Tg from an amorphous state. These PETs have slower thermal crystallization rates than unmodified PCT allowing them to be used in such processes.
In order to achieve similar crystallization rates to PET, PCT has to be modified with certain levels, approximately 15 to 30 mole % of the diol fraction coming from ethylene glycol or 15 to 30 mole % of the acid fraction coming from isophthalic acid. This results in modified PCT materials with melting temperatures (Tm) and glass transition temperatures (Tg) similar to PET (Tm=˜240° C., Tg=˜80° C.). Given the similar Tm and Tg to PET, these modified PCT materials provide little benefit in terms of heat resistance compared to PET and are typically more costly to produce.
Thus, there is a need in the art for a modification of PCT that slows down the thermal crystallization rate enough, similar to PET, to permit the molding of amorphous articles and extrusion of amorphous films that can be subsequently strain induced crystallized by orientation processes known in the art such as fiber drawing, film stretching, stretch blow molding, injection stretch blow molding and the like, to produce clear semi-crystalline articles with superior heat resistance (higher Tg and higher Tm) to clear semi-crystalline articles from PCT modified by ethylene glycol or isophthalic acid or PET.